Boiling water nuclear reactors (BWRs) typically utilize active safety systems to control and mitigate accident events. Such safety systems transport reactor decay heat to the ultimate heat sink, which is normally sea or fresh water. Active safety systems, for example, have both high-pressure and low-pressure pumping equipment. Such active systems require maintenance and surveillance testing of the safety related equipment. In addition, the pumps and other equipments typically require AC power to operate.
An alternative to an active safety system is a passive system. Totally passive safety systems have been studied for use in BWRs because of their merits in reducing maintenance and surveillance testing of the safety-related equipment, and in eliminating the need for AC power, thereby improving the reliability of BWR operation and safety. Simplified BWRs (SBWRs) have been configured to include totally passive safety features that provide more resistance to human error in accident control and mitigation.
There are, however, some tradeoffs when employing totally passive safety systems in BWRs. Due to their passive nature, the totally passive system, when configured in accordance with nuclear standards of system separation and diversity, substantially add to plant size and cost. Therefore, passive system applications to BWRs have been limited to small- and medium-sized plants having up to about 1000 MWe output.
A combination active and passive system is described in U.S. Pat. No. 5,426,681, which is assigned to the present assignee and incorporated herein, in its entirety, by reference. The system described in the above referenced patent provides many advantages, however, such system has separate passive containment cooling systems (PCCS) units and separate reactor heat removal steam condenser (RHR-CND) systems. Such separate systems are located in separate compartments of a condenser pool and add to the plant size and cost.
It would be desirable to provide a safety system for a nuclear reactor which is highly reliable and satisfies safety requirements yet has fewer safety components. For example, it would be desirable to provide PCCS and RHR-CND systems which perform the same functions as known PCCS and RHR-CND systems yet require less space and few components than such known systems.